The present invention relates to an apparatus and method for atomizing a molten liquid to form particles of substantially uniform size, particularly for the formation of relatively large metal particles of substantially uniform size with an induced duplex microstructure.
Spherical particles, and in particular spherical particles of one of a metal and a metal alloy, have increasing applications in industrial processes. Spherical particles provide good flowability, low surface area and hence a minimum of surface oxide, and efficient packing. Applications for relatively large spherical particles, approximately 200 microns to 5 mm, of uniform size, such as Thixomolding(trademark) of alloys, and other applications in ceramics, ceramic metal combinations, metals and metal alloys provide a demand which is presently not fully satisfied. Unfortunately, current practices for the formation of large particles of at least nearly spherical shape are expensive, and do not provide the level of shape, uniformity and purity demanded.
A common prior art practice for the formation of at least nearly spherical particles is disclosed in U.S. Pat. No. 4,428,894 issued to Bienvenu in 1984 in the name of Extramet. A jet of molten metal is passed through a vibrating orifice to produce a cylindrical stream of the molten metal. A cylindrical stream of such a molten metal is inherently unstable, its surface becoming increasingly perturbed as it issues from the nozzle until at some distance the stream spontaneously breaks up into separate droplets. The high surface tension of the molten metal causes the droplets to immediately assume at least a nearly spherical shape, which minimizes the surface free energy of the droplets. The spherical droplets fall from the orifice under the influence of gravity through an inert gas atmosphere contained in a cooling tower. If, however, particles larger than one millimeter in diameter are to solidify to a point where sphericity is maintained after impacting the bottom of the cooling tower, an extremely tall cooling tower is required. This cooling tower method also causes the droplets to pass through the inert atmosphere at high relative velocity, up to at least approximately 20 meters per second. High relative velocity, it has been found, distorts the spherical shape of the droplets. In addition impact with the chamber walls prior to solidification, or impact with the bottom of the cooling tower if a quench liquid is not used, flattens the particles unless the cooling tower is sufficiently tall. When quench liquids are used to remove significant latent heat, droplets that are still liquid or semi-solid can lose their spherical shape upon impact with the quench liquid. Thus even with a quench liquid, residence time in a cooling tower must still be maximized in order to permit droplets to cool sufficiently to reduce deformation.
Other factors that adversely affect particle shape include agglomeration with other droplets prior to solidification, which affects the shape and size distribution of particles. Since the individual droplets produced by the breakup of a liquid stream are irregularly shaped, a particular problem in the case of high melting point materials is that solidification can occur prior to spheroidization of the droplets, resulting in the production of irregularly shaped particles. A further problem is associated with surface oxidation. Oxides normally have a much higher melting point, and for skin-forming alloys like aluminum, this layer forms almost immediately and can make spheroidization impossible. Of course oxidation can be reduced by providing an inert gas atmosphere within the cooling tower. A drawback to this method is that since a cooling tower can be 20 meters high, circulating a cooling inert atmosphere throughout is quite expensive.
U.S. Pat. No. 4,871,489 by Ketcham, issued to Corning Incorporated in 1989, discloses the use of an inverted apparatus produced by Thermo Systems Incorporated for the production of metal oxide precursors. This apparatus is designed for the production of very fine particles, having a diameter of about 8.5 microns and not larger than 50 microns. Fluid is forced though a thin perforated plate to form a plurality of fluid streams. Oscillation of the plate is applied in the direction of the fluid flow to break up uniform droplets. The droplets are entrained in the flow of a dispersion medium, which cools and removes the light particles. However, this device is not adequate for the formation of larger particles, which have greater latent heat and kinetic energy. Sufficient cooling would not occur as particles are entrained in the dispersion fluid. The flow of dispersion fluid necessary would be rapid to lift the heavy particles from the chamber, which would adversely affect the particle shape. In addition, the greater latent heat and longer cooling time would lead to increased particle agglomeration as still molten particles contact one another in the dispersion flow. U.S. Pat. No. 4,871,489 does not teach a method for increasing the residence time for the solidification of large spherical particles.
While the prior art methods are adequate for their intended purpose of producing at least nearly spherical particles of substantially uniform size, they do not allow for any variation of the microstructure that develops during particle cooling. The particles that are typically produced by spin casting techniques are other than single crystals, and normally display some sort of grain microstructure. Often the grain microstructure of the particles is a combination of irregular xe2x80x9ccellsxe2x80x9d and dendrites. It would be advantageous to provide a method for producing metallic particles of substantially uniform size with a desired microstructure, such as for example an induced duplex microstructure. A duplex microstructure could be produced with a pure metal, for instance magnesium or aluminum, where solidification of the droplet occurs at a single temperature. Alternatively, solidification of metal alloy particles occurs over a range of temperatures. Significantly, the particular microstructure of metallic particles dramatically affects the properties of the particles, especially when the particles are subjected to thermal treatment, or even re-melted, subsequent to their fabrication. For instance, the finer grains typically melt earlier than the larger grains, which would allow for the preservation of chemical composition and relative size of the larger grains.
In order to overcome these and other limitations of the prior art, it is an object of the invention to provide a method and an apparatus for producing nearly spherical particles with an induced duplex microstructure from a jet of a molten material in an inverted stream atomization apparatus.
It is proposed to provide an inverted cooling chamber that releases a molten stream at or near the bottom to launch large particles on a parabolic trajectory having an upward and downward path. This provides a longer cooling time in a controlled atmosphere at low relative velocity without the large cooling tower currently required by the prior art. Advantageously, the lower maximum velocities that are achieved by the particles in an inverted cooling chamber allows for formation of nearly-spherical particles against a chill body for receiving the still partially molten droplets to be disposed within the particle trajectory, causing the particles to solidify rapidly with an induced duplex microstructure upon impacting the chill body.
In accordance with an embodiment of the current invention, there is provided a method of forming particles of substantially uniform size with an induced duplex microstructure in an atomization apparatus comprising the steps of: releasing a stream of molten material through an aperture under positive pressure upward into a cooling chamber where the stream breaks up into substantially spherical droplets having a kinetic energy sufficient to follow an upward trajectory above the aperture; and, allowing the droplets to impact a chill body disposed within a collection area of the cooling chamber while the droplets are at least partially molten.
In accordance with another embodiment of the current invention, there is provided a method of forming particles of inhomogeneous chemical composition and of substantially uniform size with an induced duplex microstructure in an atomization apparatus comprising the steps of: releasing a stream of molten material through an aperture under positive pressure upward into a cooling chamber where the stream breaks up into substantially spherical droplets having a kinetic energy sufficient to follow an upward trajectory above the aperture, the molten material provided to the aperture within a range of temperatures between approximately the liquidus point and the solidus point of the molten material; and, allowing the droplets to impact a chill body disposed within a collection area of the cooling chamber while the droplets are at least partially molten.
In accordance with still another embodiment of the current invention, there is provided an atomization apparatus for forming particles of substantially uniform size with an induced duplex microstructure in an atomization apparatus comprising: a vessel for containing a material at a molten state; pressurization means for applying positive pressure to at least a portion of the molten material in the vessel; a cooling chamber; at least one aperture contained in the cooling chamber communicating with the vessel for releasing a stream of the molten material under pressure upwards into the cooling chamber to break the stream up into nearly spherical droplets; at least an orifice for introducing a plume of vapor and gas coolant to impinge on the molten stream; and, a chill body disposed within a collection area of the cooling chamber for receiving the at least partially molten droplets and for providing a quench surface to rapidly solidify rapidly the at least partially molten droplets, whereby the cooling chamber further includes a top above the at least one aperture dimensioned to permit each of the droplets released to follow at least an upward path of a parabolic trajectory.